[SOLVED] problem writing over text in file

doughyi8u · 04-24-2015, 01:38 PM

I have the following program:

Code:

#include "hotel.h"

void getname();

int
addguest(void)
{
	int roomno;
	off_t offset;

	if ((roomno = findfree()) >= 1) {
		getname();
		offset = (roomno - 1) * NAMELENGTH;
		if (lseek(infile, offset, SEEK_SET) == -1) {
			perror("lseek");
			exit(1);
		}
		write(infile,namebuf,NAMELENGTH);
	} else 
		return -1;
	return roomno;
}

void
getname()
{
	int length = NAMELENGTH + 1;


	while (length >= NAMELENGTH) {
		printf("Enter Resident: ");
		scanf("s", namebuf);
		length = strlen(namebuf);
		if (length >= NAMELENGTH) {
			puts("Name is too long");
			sleep(3);
		}
	}

	/* fill buffer w/ zeros */
	for (length; length < NAMELENGTH; length++)
		namebuf[length] = ' ';
	namebuf[NAMELENGTH-1] = '\0';

	return;
}

#include "hotel.h"

const char *empty = "EMPTY               ";
char namebuf[NAMELENGTH];

int
main(int argc, char **argv)
{
	int j;
	char *p;
	infile = openfile();

	
/*	printf("Get Occupier 1:\nFree Room 2:\nAddGuest 3:\nFindFree 4:\n\n\nChoose:	*/

	if ( (j = addguest()) == -1) 
		;
	else
		printf("%s was added to room %d\n", namebuf, j);

	return 0;
}

adduser is supposed to write over the text in a line that reads EMPTY. The problem is it writes the text from namebuf pushing the EMPTY to the right. I wrote a test program that didn't do that; it wrote over the text.

Code:

ateststringralll   b
jjjjjjjjjjjjjjjjjjjj
aldkfjaldkjaldfjalk 
sfkjalkfjalkdfjaj  4
alinetotest         
DLKSJFladksjfklfdjaj
EMPTY
jlkdfldkjfdjfjjjjjjj
ldfjksllllllllllllll
EMPTY

Each line is 20 chars long including the EMPTY which has blanks tacked on the end.

I hope this is enough info to help.

NevemTeve · 04-24-2015, 01:54 PM

You cannot 'insert' data into a file (let's not mention ISAM, VSAM etc), so the problem lies elsewhere.
Add debug-prints, eg:

Code:

printf ("Debug: index=%d, index-1=%d, elem_size=%d, offset=%lld\n",
	roomno, roomno-1, (int)NAMELENGTH, (long long)offset);

doughyi8u · 04-24-2015, 01:59 PM

so it does write over? The test prog I wrote did exactly that.

michaelk · 04-24-2015, 03:12 PM

Did you post your the entire program?

Where is your function that actually opens the file i.e openfile?

infile is your file pointer but is not defined.

The big hint is understanding local vs global variables.

doughyi8u · 04-24-2015, 03:34 PM

here's the openfile function that is in a seperate file:

Code:

#include "hotel.h"

int infile;

int openfile()
{
	if ( (infile = open("residents", O_RDWR)) == -1) {
		perror("open file");
		exit(1);
	} else
		return infile;
}

And here's the main function also in a seperate file:

Code:

#include "hotel.h"

const char *empty = "EMPTY               ";
char namebuf[NAMELENGTH];

int
main(int argc, char **argv)
{
	int j;
	char *p;
	infile = openfile();

	
/*	printf("Get Occupier 1:\nFree Room 2:\nAddGuest 3:\nFindFree 4:\n\n\nChoose:	*/

	if ( (j = addguest()) == -1) 
		;
	else
		printf("%s was added to room %d\n", namebuf, j);

	return 0;
}

And here's the hotel.h:

Code:

#include <string.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>

#define NAMELENGTH 21
#define NROOMS 10

#define DBUG

char *getoccupier(int);
int openfile();
int findfree();
int freeroom(int);
int isempty(); 
int addguest(); 

extern int infile;
extern char namebuf[NAMELENGTH];
extern const char *empty;

michaelk · 04-24-2015, 04:01 PM

You forgot to post the findfree function.

doughyi8u · 04-24-2015, 04:50 PM

Code:

#include "hotel.h" 

int
findfree()
{
	off_t offset;
	int j;
	ssize_t nread;

	for(j = 1; j <= NROOMS; j++)
	{
		offset = (j-1) * NAMELENGTH;
		if ( lseek(infile, offset, SEEK_SET) == -1) {
			perror("lseek");
			exit(1);
		} else {
			if ( (nread = read(infile, namebuf, NAMELENGTH)) <= 0) {
				perror("read?");
				exit(1);
			} else 
				namebuf[nread-1] = '\0';
		}
		if ( (isempty()) == 1) 
			return(j);

	}
	puts("No empty rooms");
	return -1;
}



int
isempty()
{
	if ( (strncmp(namebuf, empty, strlen(empty))) == 0) 
		return 1;
	else 
		return -1;
}

michaelk · 04-24-2015, 09:01 PM

Actually each line is 21 bytes. So it appears that the name is being tacked on to the end but actually the line feed is being deleted. write(infile,namebuf,NAMELENGTH); writes 21 bytes, namebuf[21] is never set and the 21 byte of the line is a \n.

I will give you a hint that the getname function has a syntax error somewhere in the code you posted.

doughyi8u · 04-24-2015, 09:33 PM

I can't find the error in getname()

michaelk · 04-24-2015, 09:40 PM

One more hint. With the posted code namebuf is never actually being assigned the name entered.

doughyi8u · 04-24-2015, 09:51 PM

is it in the while or for loop? I still can't find it.

michaelk · 04-24-2015, 09:53 PM

Is the residence name actually being written to the file?
Check the syntax for scanf("s", namebuf);

doughyi8u · 04-24-2015, 09:58 PM

it is being written to file but pushes what was on that line to the write extending the limit of 20 chars
the scanf("s",namebuf) function was a typo. the program reads the way it should be: scanf("s",namebuf);

michaelk · 04-24-2015, 10:02 PM

Nope.
http://www.tutorialspoint.com/c_stan...tion_scanf.htm

doughyi8u · 04-24-2015, 10:10 PM

I used the syntax scanf("s",&namebuf) and get the same results as w/o the & in front of the buffer